In the field of electrochemistry there is a well-known electrochemical cell known as a chlor-alkali cell. In this cell, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industries are produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION.RTM. manufactured by the E. I. du Pont de Nemours and Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode.
One of the unwanted byproducts present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
It has been estimated that 25 percent of the electrical energy required to operate a chlor-alkali cell is utilized due to the formation of hydrogen at the cathode. Hence, the prevention of hydrogen formation, e.g., by reacting water with oxygen at the cathode resulting in the formation of hydroxide, can lead to substantial savings in the cost of electricity required to operate the cell. In fairly recent attempts to achieve cost savings and energy savings in respect of operating chlor-alkali cells, attention has been directed to various forms of what are known as oxygen (air) cathodes. These cathodes prevent the formation of molecular hydrogen at the cathode and instead reduce oxygen to form hydroxyl ions. Savings in cost for electrical energy are thereby achieved.
One known form of oxygen (air) cathode involves use of an active layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed (enhanced) using precious metal catalysts, such as silver, platinum, etc. Unfortunately, however, the pores of such active carbon particles may become flooded with the caustic soda thereby significantly reducing their ability to catalyze the reduction of oxygen at the air cathode and resulting in decreased operating efficiency. In an attempt to overcome these difficulties in flooding of the active carbon, hydrophobic materials, e.g., polytetrafluoroethylene (PTFE), have been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the active carbon layer, per se, and/or to a protective (wetproofing) or backing sheet which can be laminated or otherwise attached to the active layer. Thus, PTFE has been employed in both active layers and in backing (wetproofing) layers secured thereto. Such active carbon-containing layers, however, are subjected to loss of strength resulting in failure combined with blistering thereof when the chlor-alkali cell is operated at high current densities, viz., current densities of about 250 milliamperes/cm.sup.2 and higher for prolonged time periods.
In the fuel cell art, the cathode of fuel cells can be made of conducting particles, e.g., conducting particles of carbon, for example, active carbon, viz., carbon having a B.E.T. surface area greater than 600 m.sup.2 /g. Catalyzed active carbon is a suitable catalyst for the reduction of oxygen to hydroxide at the cathode.
In such cathode active layers, which contain the active carbon catalyst particles, there is encountered a situation whereby the caustic soda (NaOH) wets the catalyst surface and in essence blocks the pores from contacting the reactants and thereby functioning as a catalyst. Additionally, the active carbon particles are corroded when in use, and this corrosion reduces their catalytic efficiency and also their surface area.
Various techniques have been developed in the art to reduce the wettability of the active carbon catalyst particles. One of these techniques involves mixing or otherwise incorporating polytetrafluoroethylene (PTFE) particles with the active carbon particles to impart hydrophobicity to the active carbon layer and lessen or inhibit its disposition to be wetted by the alkali present in the chlor-alkali cell. Since the PTFE particles are less electrically conductive than the active carbon particles, they detract from the overall conductivity of the active layer. Hence, it is difficult to obtain a balance between hydrophobicity, catalyst activity, retention of surface area and reduction in electric power requirement for conducting this electrochemical reaction which takes place in chlor-alkali cells.
The present invention enables the attainment of a very desirable combination of balanced properties in the active layer of an oxygen cathode which is useful in a chlor-alkali cell thereby resulting in savings which accompany prevention of hydrogen production at the cathode in a chlor-alkali cell. It has been observed that the partially fluorinated CF.sub.x =0.1 to about 0.18 active carbon particles, both catalyzed and uncatalyzed, can be used in accordance with this invention when incorporated into the active layer of an oxygen (air) cathode.